1. Field of the Invention
This invention relates to to a driving assembly comprising a drive member, a driven member, a viscous shear coupling and a clutch between the driven member and the viscous shear coupling and selectively operable to allow torque to be transmitted from the drive member to the viscous shear coupling and from the viscous shear coupling to the driven member.
Such a drive assembly finds particular application in a drive transmission for a motor vehicle in which there is a prime mover and a drive line between the prime mover and the road wheels. The drive line can include a driving assembly as described so that the drive may be selectively applied to the road wheels through the driving assembly.
More particularly, the arrangement is useful in a four-wheel drive vehicle in which one set of road wheels is directly driven from the prime mover and the other set of road wheels is driven through a driving assembly as described only when the directly driven wheels begin to slip.
2. Description of Prior Art
It is known in a four-wheel drive vehicle to have one set of road wheels directly driven from the prime mover and the other set of road wheels driven via a viscous shear coupling. Such a coupling may, for example, comprise a housing and a hub with the hub being rotatably mounted in the housing. There are two sets of plates, the plates of one set being carried by the housing and the plates of the other set being carried by the hub. These plates are interleaved and the housing contains a viscous liquid such for example as a silicone oil. Such a viscous coupling is shown, for example, in U.S. Pat. No. 3,760,922.
With such a coupling, torque is transferred between the housing and the hub when there is a relative speed difference between them as a result of shearing of the silicone fluid between adjacent plates. As is mentioned above this drive may be used to drive one set of wheels when the directly driven wheels tend to slip.
In the prior art, where a viscous shear coupling is interposed in the drive line to a set of wheels, torque is transmitted to those wheels whenever a speed difference occurs between the two sets of wheels. However disadvantages can occur where the vehicle is equipped with anti-lock braking.
Moreover, if it is the front wheels that are directly driven and if these tend to lock on braking so that a speed difference occurs between the front and rear wheels, torque can be transmitted by the viscous shear coupling to the rear wheels while the front wheels are locked; this tends to make the vehicle unstable.
Various proposals have been made to solve this problem by disconnecting the viscous coupling so there is no tendency to drive the rear wheels. Thus if the rear wheels overrun the front wheels there is in effect a free wheel device. It has also been proposed that when the brakes are applied the drive to the viscous coupling is disconnected.